Photoreceptors (rods vs cones) | Processing the Environment | MCAT | Khan Academy


Let’s examine the difference
between rods and cones in our eyes. Let me draw a very simplified
schematic of a rod just to give you an idea
of what it looks like. So rods actually get their name
because if you look at a rod under a microscope, it actually
has this elongated cell body that kind of
gives it a rod shape. So a rod is a photoreceptor. What exactly is a photoreceptor? Is it a neuron? Is it a type of nerve? So, in fact, it is. It’s a very specialized
type of nerve that’s able to take in
light and convert it into a neural impulse. So inside a rod, there
are a whole bunch of structures known
as optic discs. And these optic discs are
large, membrane-bound structures inside the rods. And there are thousands of
them in an individual rod. Embedded within the
membrane of each optic disc is a whole bunch of proteins,
and these proteins actually absorb light and begins a
phototransduction cascade that eventually leads this rod to
fire an action potential that will reach the brain. Similarly, a cone gets its
name because it’s cone-shaped. Cones are also photoreceptors. So they’re specialized
nerves that have the same internal
structure as a rod. So cones also have a whole
bunch of these optic discs that are stacked upon one
another, and embedded within each optic disc is a
whole bunch of this protein. So as I mentioned over
here, the protein in a rod is known as rhodopsin. In cones, it’s basically
the same protein. But it just has another name,
and it’s called photopsin. So as I mentioned, as a ray
of light enters the eye, if it happens to hit a rod, and
it happens to hit rhodopsin, it’ll actually trigger the
phototransduction cascade that results in this rod
firing an action potential. This exact same process
happens in a cone. So these are the
major similarities between rods and cones. Now let’s look at
the differences. So in an average retina,
there about 120 million rods. In contrast, there about 6
million cones per retina. So there are about
20 times more rods than there are
cones in each eye. Another big difference
between rods and cones is where they are
located in the eyeball. So if I draw a very simplified
diagram of an eyeball, and this is the optic nerve exiting
the back of the eye. So this would be the
front of the eyeball. This is the back of the eyeball. And as I mentioned in a previous
video, the back of the eyeball is coated by a membrane
known as the retina. So rods are actually found in
the periphery of the eyeball. So they’re found in
this area over here and in this area over here. And there’s actually a region
of the retina, right about here, that actually dimples in. And this region is
known as the fovea, and cones are
mostly concentrated in this region in
front of the fovea. So rods are mostly found in
the periphery of the eye, whereas cones are mainly
found near the fovea. Another big difference
between rods and cones is that rods do not produce
color vision, whereas cones do. So rods are very
sensitive to light. In fact, they are
1,000 times more sensitive to light
than codes are. For this reason, rods are
really good at detecting light. So they’re basically
responsible for telling us whether or not light is present. Another way to
think of this would be black and white vision. On the other hand, cones
are not as sensitive. But they do result in
the detection of light. So they result in color vision. And in fact, there are three
different types of cones. So there are red cones,
which make up about 60% of all cones in the eye. There are green cones,
which make up about 30% of all cones in the eye. And there are blue cones,
which make up about 10% of all cones in the eye. Another major difference
between rods and cones is their recovery time. So rods have a very
slow recovery time, whereas cones have a
very fast recovery time. So what I mean by slow
and fast recovery times is that as soon as a rod is
activated by a ray of light– so let’s imagine
that a ray of light comes in and activates
this rod, and it fires an action potential. It takes a lot
longer for the rod to be able to fire another
action potential than it does for a cone, and you’ve
actually experienced this. So if you’ve ever been outside,
playing soccer or football, and you run inside to
get a cup of water, there’s a big change
in illumination, yet you don’t stub your toe. You’re able to
transition from outside to inside really quickly. That’s because cones are
able to rapidly adapt to changes in illumination,
whereas rods take a lot longer. So at night, when you
walk into a dark room, it takes a while for your eyes
to get adjusted to the dark. And that’s because the rods
need to be reactivated, reset, in order for you to be able
to use them to see anything.

27 thoughts on “Photoreceptors (rods vs cones) | Processing the Environment | MCAT | Khan Academy

  1. "Rods and cones do not fire action potentials"  —Edwin Relf
    How do you respond to that?

    Also, isn't the slow recovery time due to pupil dilation/contraction?
    What I'm getting at is that isn't there a time limit(refractory period), which, once elapsed, any neuron will necessarily fire an action potential, regardless of what kind of neuron it is, given the right conditions(more permeability for Na+ than K+)?

  2. I don't know if Khan academy has already done this or not, but you should make a video on the visual pathways (Geniculostriate, tectopulvoinar, and Retinohypothalamic). It is a bit hard to understand just by reading about them from the textbook.

  3. What is the difference between photopsin and iodopsin? My book says that cones contain iodopsin. Are they just two words for the same thing?

  4. Looks like the video (at the end) is also incorrect about dark vs. light adaptation (they got it backward). Bummer. Was going to use this video to help my students but there's too many conflicting details and I don't want to confuse them.

  5. Aren't Rhotopsin snd Photopsin the pigments that are broken down that stimulate the generator potential?

  6. I spent a solid hour of searching to find this exact explanation, and then I read the comments and key parts are wrong?? If this is not accurate you should delete or edit the video. You're misinforming people. If you have other videos, you should fact check them and make sure you're correct when stating scientific facts.

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